The present application relates generally to a receiving antenna and, more particularly, to a fiber optic receiving antenna that does not re-radiate the incident power.
Conventional antennas that are designed to receive signals also tend to re-radiate at least some of the incident power. In this regard, in responding to an incident electric field, such as the incident electric field created by radio frequency (RF) signals, current will be induced in a conventional antenna. This induced current will generally be conducted away from the antenna through a transmission line to a receiver. Unfortunately, the induced current will also cause at least a portion of the incident power to be re-radiated. Accordingly, a conventional receiving antenna can be characterized by an effective aperture which receives the incident signals and a scattering aperture which is a measure of the incident power that is re-radiated by the current induced within the antenna. Under impedance matched conditions a conventional receiving antenna re-radiates about one-half of the incident power.
In some commercial applications, such as instrumentation applications, the re-radiation of a portion of the incident power by a receiving antenna is disadvantageous since the re-radiated portion of the incident power may perturb the electric field that is being measured, thereby impairing further measurements of the electric field. Since conventional antennas are formed of one or more conductive elements, conventional antennas are also disadvantageous in applications in which the antenna will be positioned near a power line or some other electrical hazard.
In addition, antennas are often mounted upon aircraft, including low observable (LO) aircraft. For at least those antennas mounted upon LO aircraft, the re-radiation of any portion of the incident power is disadvantageous. In this regard, if a receiving antenna that is mounted upon a LO aircraft re-radiates even a relatively small portion of the incident power, the radar cross section of the aircraft can he seriously compromised. Under stealthy operating conditions, an LO aircraft will not intentionally transmit or otherwise radiate RF signals. However, it would be desirable to continue to receive RF signals, at least for the purposes of radar warning and signal intelligence, even in instances in which the LO aircraft is operating under stealthy conditions. Unfortunately, the re-radiation of any portion of the incident power will tend to disadvantageously compromise the radar cross section of the aircraft if the aircraft attempts to receive signals when operating in a stealthy condition.
A fiber optic receiving antenna is therefore provided that permits the reception of an incident electric field without re-radiating any portion of the incident power. As such, the fiber optic receiving antenna is particularly advantageous for those applications in which the re-radiation of at least a portion of the incident power is disadvantageous, such as receiving antennas mounted upon LO aircraft and receiving antennas utilized in commercial applications that do not wish to perturb the incident electric field.
The fiber optic receiving antenna includes a master oscillator for generating a reference signal having a predetermined amplitude and frequency. The fiber optic receiving antenna also includes a fiber optic interferometer. The fiber optic interferometer includes first and second arms formed of respective optical fibers. Each arm receives the reference signal generated by the master oscillator and supports the propagation of the reference signal therealong. According to the present invention, at least one of the optical fibers is electrooptically active. In addition, the respective electrooptic activity of the optical fibers that form the first and second arms differ. As such, the signals propagating along the first and second optical fibers will be modulated differently in response to an incident electric field. In one embodiment, the optical fiber that comprises the first arm is polarized differently than the optical fiber that comprises the second arm. For example, the optical fiber that comprises the first arm can be polarized and the optical fiber that comprises the second arm can be non-polarized. Alternatively, the optical fiber that comprises the first arm can be positively polarized and the optical fiber that comprises the second arm can be negatively polarized.
The fiber optic interferometer also includes a coupler for combining the signals that have propagated along the first and second arms and been differently modulated by the incident electric field. In addition, the fiber optic receiving antenna can include a detector, such as a heterodyne detector and, more preferably, a double balanced coherent receiver, for receiving the combined signals from the coupler and for generating a signal that is at least partially dependent upon the incident electric field. As such, a measure of the incident electric field can be determined by the fiber optic receiving antenna of the present invention. However, the fiber optic receiving antenna of the present invention does not generate any current and, therefore, does not re-radiate any portion of the incident power.
The fiber optic receiving antenna of one advantageous embodiment also includes a phase modulator disposed within one of the first and second arms for tuning the fiber optic interferometer. In this embodiment, the fiber optic receiving antenna can also include a photodetector for receiving the combined signals from the coupler and for driving the phase modulator in response thereto. In this regard, the coupler typically generates first and second outputs based upon the combined signals. The photodetector of this embodiment can therefore receive the second output and output a photocurrent that is used to drive the phase modulator so as to reduce and, more preferably, eliminate, the second output in the absence of an incident electric field. By reducing the second output in the absence of an incident electric field, the fiber optic receiving antenna of this embodiment increases the percentage of the combined signals delivered to the first output to which the detector is responsive, thereby permitting the fiber optic receiving antenna to obtain a more accurate measure of the incident electric field.
According to one embodiment, each optical fiber is designed to preferentially respond to an incident electric field that is oriented at a predetermined angle relative to the optical fiber. By designing each optical fiber so that the anticipated direction of propagation of the incident electric field will be oriented so as to cause the optical fiber to preferentially respond, the differences in modulation introduced by the first and second arms of the fiber optic interferometer are increased, thereby further improving the measure of the incident electric field obtained by the fiber optic receiving antenna.
Accordingly, the fiber optic receiving antenna of the present invention detects an incident electric field and obtains a measure thereof without re-radiating any portion of the incident power. As such, the fiber optic receiving antenna is particularly desirable for commercial applications that do not wish to perturb the incident electric field which, in turn, could disadvantageously alter further measurements. In addition, the fiber optic receiving antenna of the present invention can advantageously be mounted upon or within the skin of an LO aircraft so as to permit signals to be received without re-radiating any portion of the incident power which could otherwise compromise the radar cross section of the LO aircraft.